The present invention relates generally to sensor construction and use in communication systems and, in an embodiment described herein, more particularly provides a hydrophone for use in a downhole tool.
Many applications exist for hydrophones and other pressure pulse sensors. For example, in the downhole environment, a hydrophone may be used in a tool to receive signals transmitted as pressure pulses from the surface, a sensor may monitor seismic signals that create pressure waves in a wellbore, a drill string may include a sensor to monitor hydrostatic pressure waves during drilling, etc. Of course, applications exist in other environments as well.
Unfortunately, conventional hydrophones and other pressure sensors are typically somewhat fragile, do not respond well to low frequency pressure waves and are sensitive to movement of the tools carrying the sensors. The fragility and tool movement sensitivity problems are undesirable in any environment, but are particularly detrimental in the downhole environment where tool movement, shock and vibration, temperature extremes, etc. are common. Additionally, where a pressure sensor is used in a downhole signal transmission system, the lack of low frequency response is very undesirable since it is known that pressure pulses are attenuated far less at low frequencies and, therefore, low frequency signals, may be transmitted greater distances. Thus, it would be a significant improvement in the art to provide a pressure sensor that is robust, is insensitive to movement of the tool carrying the sensor, and which has enhanced low frequency response.
Hydrophones used in downhole tools are usually each contained in a fluid-filled chamber, which is isolated from well fluids by a floating piston. Well fluids are typically conductive and sometimes corrosive, acidic, or otherwise harmful to sensors, and so the floating piston is used to separate the well fluids from the hydrophone sensor. The fluid contained in the chamber about the sensor is typically an inert oil, such as silicone oil.
This configuration, wherein a floating piston separates well fluids from oil in the sensor chamber, has several drawbacks. Maintenance of the sensor is inconvenient, since the chamber must be filled with the oil and evacuated of air each time the sensor is disturbed. There is a requirement that the special oil be available each time the sensor is serviced. Additionally, the floating piston must displace to transmit a pressure pulse thereacross and may hinder the detection of low frequency pressure pulses by the sensor, due to the mass of the piston and the friction between its seals and the bore in which it reciprocates.
Therefore, it may be seen that it would be very desirable to provide an improved and more convenient method of isolating a sensor from well fluids. Furthermore, it would be very desirable to enhance the low frequency response of a pressure sensor while obtaining the improved isolation from well fluids.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a hydrophone is provided which includes multiple piezoelectric crystals arranged in a stack. Methods associated with improved pressure sensors are also provided.
In one aspect of the present invention, a pressure pulse sensor is provided which includes at least one lead titanate piezoelectric crystal. The crystal is sensitive to axial forces applied thereto, but is relatively insensitive to lateral forces. The crystal is, therefore, insensitive to lateral accelerations of the fixture or tool holding the sensor. Preferably, the crystal is generally disc-shaped.
In another aspect of the present invention, a stack of piezoelectric crystals are used in a pressure pulse sensor. The crystals may be axially aligned and may be adhered to each other to thereby permit transmission of tensile forces therebetween. Acceleration of a tool in which the sensor is carried will preferably create tension in one portion of the crystal stack and compression in another portion of the stack, when the acceleration is along the axis of the stack. In this manner, the output of the crystals in tension due to the acceleration will cancel the output of the crystals in compression due to the acceleration, thereby eliminating any contribution of the tool movement to the sensor output.
In a further aspect of the present invention, the stack of piezoelectric crystals are mounted to a tool so that acceleration of the tool along an axis of the stack produces compressive forces in one portion of the stack and tensile forces in another portion of the stack. In several described embodiments, a mounting portion of the sensor is aligned with a center of mass of the crystal stack. When the center of mass of the crystal stack is accelerated along the stack axis by the mounting portion, one portion of the stack is in compression and another portion, of the stack is in tension.
In yet another aspect of the present invention, a membrane may be used to isolate one or more piezoelectric crystals of a sensor from fluid surrounding the sensor. Preferably, the. crystals are in direct contact with the membrane and the membrane completely encloses the crystals. The membrane does, however, permit transmission of fluid pressure pulses from the fluid to the crystals.
In still another aspect of the present invention, a membrane enclosing one or more piezoelectric crystals of a sensor is sealed to a bulkhead. At least one conductor extends outwardly from the crystals, through the membrane and into the bulkhead. The membrane may apply a compressive force to the bulkhead at a circuitous path formed on the bulkhead. Additionally, the membrane may extend into a passage formed in the bulkhead through which the conductor extends, and the membrane may be mixed with an insulating substance in the passage.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention hereinbelow and the accompanying drawings.